Internal electrode resistance plays a role affecting the power capability of electric double-layer capacitors. Capacitor cells made of activated carbon cloth are analyzed using ac impedance spectroscopy. To reduce the internal resistance, carbon nanotubes (CNTs) are grafted on the carbon cloth via sputtering catalyst seeding, followed by chemical vapor deposition with CH4. The impedance spectra of the assembled capacitor cells showed that the internal resistance of the electrodes was significantly reduced from 0.9 to 0 Omega. In addition to the increase in the electronic conductivity, the introduction of CNTs also enhanced the utilization factor of the carbon electrode, thus leading to a more efficient double-layer formation inside carbon micropores. Constant phase element analysis of the capacitive behavior in micropores showed that the deviation from ideality was less significant for the carbon cloth grafted with CNTs. This developed CNT-grafting technique significantly promoted the power performance of carbon cloth by enhancing the transport of both electrons and ions in the assembled capacitor cells. At a current density as high as 150 mA cm(-2), a capacitance loss of only 7% (compared to the ultimate value) was obtained for the CNT-grafted carbon cloth. (C) 2008 The Electrochemical Society.